[BUG] branches stats fails if there are no branches in the repo

### API Spec link

https://learn.microsoft.com/en-us/rest/api/azure/devops/git/stats/list?view=azure-devops-rest-7.1&tabs=HTTP

### API Spec version

7.1-preview.1

### Describe the bug

I have a git repository with no branches.
Calling `{organization}/{project}/_apis/git/repositories/{repositoryId}/stats/branches` returns with a 400 error.

### Expected behavior

An empty list is returned. E.g.:
```
{
"count": 0,
"value": []
}
```

### Actual behavior

An error is returned:
```
{
"$id": "1",
"innerException": null,
"message": "VS403403: Cannot find any branches for the reposynctest repository.",
"typeName": "System.ArgumentException, mscorlib",
"typeKey": "ArgumentException",
"errorCode": 0,
"eventId": 0
}
```

### Reproduction Steps

- Create a repository.
- Delete all branches.
- Call the endpoint.

### Environment

_No response_

Start by reproducing the Azure DevOps Git stats branches endpoint at `{organization}/{project}/_apis/git/repositories/{repositoryId}/stats/branches` after deleting all branches, using the linked API specification as context. Done means the endpoint returns HTTP success with `{ "count": 0, "value": [] }` instead of a 400 error.

Written by the indexing model from the issue text.